The methods most commonly used in producing SiC single crystals are sublimation techniques based on the Lely method, which are realized by vapour-phase crystallization, as a result of evaporation of solid silicon carbide (U.S. Pat. No. 2,854,364; U.S. Pat. No. 4,866,005). As shown in (S. Yu. Karpov, Yu. N. Makarov, M. S. Ramm, R. A. Talalaev "Excess Phase Formation during Sublimation Growth of Silicon Carbide". Presented at the 6th International Conference on Silicon Carbide, September, 1995, pp. 73-74, Kyoto, Japan), the SiC monocrystal growth out of vapour, without forming any secondary-phase inclusions, is only realized if the external silicon (Si) flux onto the growing surface exceeds the carbon (C) flux.
The required excess silicon flux is dependent on the temperature of the growing surface and determined, in the case of sublimation technique, by composition of the vapour adjacent said surface of growth. The conditions of the single-phase (i.e. without secondary phase inclusions) monocrystalline SiC growth are met with the vapour composition approximating the SiC--Si system (Spravochnyik po elektrotekhnicheskim materialam, Ed. by Yu. V. Koritsky, V. V. Pasynkov, B. M. Tareyev, Energoatomizdat/Leningrad/, 1988, p.449). It has been shown by experiment that non-observance of this condition results in a sharply increased density of dislocations, channels and micropipes in the growing crystal. Since it is the silicon molecules that have the maximum concentration in the gaseous phase, any drift of the substance from the growth zone will result in the vapour phase within the growth zone being depleted of silicon, and hence enriched with carbon, thus ultimately leading to graphitization of the source, degradation of the crystal quality, discontinuation of the growth process. The shift of the vapour composition, in the growth zone, towards the vapour phase corresponding to the SiC--Si system substantially improves the growth and contributes to a more perfect structure of the SiC single crystal grown. This is due to the fact that such system (SiC--Si) prevents the secondary phase of graphite from being generated, avoiding graphitization of the source and the growing surface of the seed crystal. It is also known, however, that excessive silicone in the growth zone may results both in the formation of defects on the growing surface of the SiC crystal, due to the precipitation thereon of excess silicon drops, and in generation of polytypes differing from the seed polytype.
In the patents U.S. Pat. No. 2,854,364; DE 3,230,727, it is proposed that SiC powder of a predetermined granularity, with a mass of more than three times the mass of the single crystal to be grown, be placed in a lump, in the growth zone, in order to maintain the required vapour phase composition for a certain time, the powder serving as the source of silicon carbide vapours. In this case, a relatively constant composition of the vapour phase within the growth zone is provided, because the drift of SiC vapours into the space outside the growth zone, which might have resulted in a continuous enrichment of the vapour phase with carbon atoms, is balanced out by an abundant generation of SiC vapours, since the vapour phase is substantially more enriched with silicon atoms than with carbon atoms. This enables a relative stability of the vapour phase composition in the vicinity of the growing surface of the SiC single crystal to be maintained for a certain period of time. The duration of the stable growth process, however, is limited, and so the SiC--C system comes to be realized, with time, in most of the volume of the growth zone, which has been previously shown as being detrimental to the growth process. The large amounts of the SiC powder consumed leads to an increased cost of the grown single.
In U.S. Pat. No. 4,866,005 a technique has been proposed which allows an essentially unlimited duration of the growth process by continuously feeding specified small portions of SiC powder of a given granularity into a predetermined temperature zone of the growth chamber. However, in this case, again, the mass of the material consumed in the SiC vapour source will be much in excess of that of the single crystal grown, due to the SiC vapours being removed to the space outside the growth zone, for the growth chamber communicates with the environment, making this method, like the methods of U.S. Pat. No. 2,854,364; DE 3,230,727, uneconomical. The loss of the SiC source material is also caused by the growth zone geometry, and particularly, by a relatively large separation (about 10 cm) of the evaporating surface of the SiC vapour source and the seed growing surface, which by far exceeds the Si, Si2C, SiC2 molecule track length at the working pressure in the growth zone.
The method most closely approximating that herein proposed is the sublimation technique of growing SiC single crystals, as disclosed in the patent U.S. Pat. No. 4,147,572 (the so-called "sandwich-method"). According to this disclosure, the evaporating surface of the SiC source and the growing surface of the SiC seed crystal are arranged in parallel, at the distance not exceeding 0.2 of the maximum linear dimension of the source, to form the growth zone. The single crystals are grown in a graphite crucible in an inert gas atmosphere, at temperatures of 1600 to 2000.degree. C., with an axial thermal gradient of 5 to 200.degree. C./cm, in the direction from the seed crystal to the source. With small gaps between the SiC source and the seed crystal, the loss of SiC vapours from the growth zone can be substantially reduced and their flow directed along the straight path from the source to the growing surface of the seed crystal. The growth zone is here screened from external impurity sources, resulting in a reduced concentration of impurities in the single crystal grown. In addition, the small gap between the evaporating surface of the source and the growing surface of the seed, in comparison to the size of the SiC source, allows a uniform temperature along their surfaces to be maintained and the temperature difference between them controlled.
This method suffers from a number of drawbacks. One of them is the small volume of the single crystals grown (less than 1 mm thick) due to a sharp drop in the growth rate, as the crystallization time increases, as a result of the silicon at the edge of the growth zone being volatilized beyond its bounds and consequently, excessive carbon released from the evaporating surface of the SiC source and the growing surface of the grown crystal, slowing down the growth process. In this case, the single crystals obtained by the above technique show defects such as secondary-phase inclusions (predominantly, graphite), micropipes with a density of more than 100 per cm.sup.2 and dislocations numbering at least 10.sup.-4 per cm.sup.2. They are also relatively inferior, as regards the concentration of residual impurities such as boron, oxigen, etc.
Known in the art is a favourable effect of tantalum (Ta) on the growth of monocrystalline SiC. In particular, theoretical investigations of the sublimation growth of monocrystalline SiC with a "sandwich-cell" in a tantalum container have shown that the vapour medium produced in the growth zone is close to the SiC--Si system with the partial silicon vapour pressure slightly exceeding the pressure in the SiC--Si system (D. Hofmann, S. Yu. Karpov, Yu. N. Makarov, E. N. Mokhov, M. S. Ramm, A. D. Roencov, Yu. A. Vodakov, "The Use of Tantalum Container Material for Quality Improvement of SiC Crystals Grown by the Sublimation Technique" Presented at the 6th International Conference on Silicon Carbide, September, 1995, p.15, Kyoto, Japan). Here, the favourable effect of tantalum is made evident when the process is run both in the inert gas atmosphere and in vacuum.
It was found by the authors, however, that at an early stage of the growth process, tantalum or its compounds may be extracted as secondary-phase inclusions, and an increased concentration of dissolved tantalum in the monocrystalline SiC being grown may occur, whose presence as a dopant in the single crystal is not desirable. And even in the presence of tantalum, with more than 5-hour duration of the growth process run in accordance with the above technique, the quality of the single crystal grown is gradually impaired by the embedment of carbon dust deposited on the growing crystal. The latter circumstance is caused by the aforementioned mechanism of carbon enrichment of the vapour phase due to silicon drift to the outside of the growth zone. This is accompanied by losses of the source material amounting to 15-20% of the weight of the substance transferred.
Further, at the initial stage of the growth process, the silicon vapours formed in evaporating the SiC source may interact with the material of the tantalum container, resulting in the formation of a low-melting-point tantalum-silicon alloy and, as a consequence, in the destruction of the container at the working temperatures of the growth.
In known sublimation methods of growing silicon carbide single crystals, the SiC vapour source may be either a silicon carbide powder of the specified dispersity, presynthesized from a spectrally pure silicon/graphite mixture, or a poly- or monocrystalline SiC wafer produced, for example, by the Lely method (E. N. Mokhov, M. G. Ramm, Yu. A. Vodakov "Vyrashchivanie epitaksialnykh sloev karbida kremnia". Vysokochistie veshchestva. No.3, 1992, pp. 98-105). In this case, in order to obtain a large amount of the grown single crystals, the SiC vapour source is required to be continuously fed to the growth area. The use of silicon carbide powder as the SiC source is more economical than that of poly- or monocrystalline silicon carbide, which is much more expensive than the powder. The continuous supply of the powder into the growth zone, however, is rather a complicated problem.
In addition, as the SiC powder is synthesized, coarse-sized impurities such as graphite or other dust are generally entrapped by the SiC powder grains. As the SiC single crystals grow, dust is transported to the growing surfaces along with SiC molecules, substantially impairing the perfection of monocrystalline structure and resulting in a high density of micropipes and dislocations in the crystal grown. Thus, the quality of the grown silicon carbide may be insufficient to serve as the basis for semiconductor devices.
The nearest analogy to the proposed invention is the use of silicon carbide poly- or monocrystals as the SiC source (U.S. Pat. No. 4,147,572). As such SiC sources are free from coarse-grained impurities, higher-quality SiC single crystals result. As was mentioned above, however, the poly- or monocrystals of silicon carbide are rather expensive, making their use as the SiC source uneconomical.